Parameters in high current transmission in conventional conductors like Cu, Al, etc. are mostly due to the resistance of the conductors, which produces a significant amount of energy loss. Loss-less transport of current has been the main point of attraction in superconductors from the very beginning. The low Tc conventional superconducting materials have been used in making high field magnets (˜20 tesla), which are now readily available. The operative temperature, i.e. 4.2K (requiring a constant flow of liquid of helium) has hampered the growth of high current carrying leads and cables. The advancements in this field of high transport current are restricted up to prototype level only. The advent of high Tc superconducting compounds (HTSC) has raised the hopes for high transport current leads as the operating temperature is raised to 77K. However, poor ductility and low critical field had put restrictions on immediate applications. However the development of HTSC multifilamentary cables of Bi(2223) having Jc˜105 A/cm2 at 77K in a field 0.6 T showed a good promise but again they are not a very conventional answer. In comparison, the tube conductors of high Tc superconductors have shown good potential. HTS current-leads based on BSCCO tubes and rods are the first applications of ceramic superconductors in electrical power engineering where they offer a major advantage over both, the conventional superconductors as well as the traditional all-metal leads of good conductors such as Cu and Al. Conventional low Tc superconductors embedded in copper were considered a better option over all-metal leads because of their zero resistance and their capacity to transport high currents defined by their critical current density Jc (˜105 Amps/cm2), but because of the restrictions of operation at 4.2K these materials could not always replace Cu or Al.
In addition to the higher operational temperature of 77K, the HTSC materials have their low thermal-conductivity, which reduces the heat-losses by more than a typical factor of 10. This reduces the heat load on the cryogenic system and results in a significant reduction in refrigeration cost and allows for new innovative cooling concepts. Their other applications are in the field of magnetic shielding and current-limiters.
All above utilization of HTSC tube conductors for high current application (Ic>1000 A) become ineffective and lossy if the contact resistance of the joints of the normal conductors (Cu, Al) feeding high currents to HTSC tube conductors are of the orders of 10−4-10−3Ω. The requirement to utilize the tube conductors to their optimum, the contact resistance of the normal conductors joining to HTSC tube conductors should be at least of the order of 10−6Ω.
Reference may be made to disclosure by K H Sandhage et al. in Journal of Materials Vol 43, pp 21 (1991) wherein it is taught that among the HTSC family the Y-based superconductors suffer from many crystallographic limitations to synthesize tube and rod conductors and only thin film applications are commercialized. In yet another disclosure by E H Hellstorm in Materila Research Bulletin Vol XVII, pp 45, (1992) it has been taught that T1 based superconductors due to health hazards are not being used for bulk applications. Only Bi-based superconductors (Bi2−xPbxSr2Ca2Cu3Ox) and Bi(2212) are commercially economical and suitable answers as reported by S X Dou and H K Liu in Supercond. Science and Technol Vol 6, pp 297, (1993).
The contact resistance problem for high current electrical connection can only be solved using silver as normal conductors feeding the high current to Bi-based tube conductors. The major problem is to connect the silver feeder to Bi(2223) ceramic surface.
The problem has been tackled in parts in several ways:
U.S. Pat. No. 5,149,686 and a US Pat (publication No 20030132023) disclose sputtering the non-superconducting metal (Ag, Au) on small bar shaped HTSC of μm order for making the electrical contact.
Plasma spray technique of Ag/Au film on HTSC of μm order has been disclosed by Y Yamada in Bismuth Based High Temperature Superconductors Ed by H Maeda and T Togano pp 277 (1996).
Then the high current feeders have been soldered on HTSC surface and a contact resistance of the order of 10−6Ω has been achieved at 77K.
For small samples the sputtering technique has been successful but plasma deposition is used specially for bismuth based large samples like tube/rod conductors.
U.S. Pat. No. 5,506,199 and K K Michishita et al in Bismuth Based High Temperature Superconductors Ed by H Maeda and T Togano pp 253 (1996) disclose a process by partially encasing Ag tube, sheet or wire in large samples of Bi 2212 melt.